Method of applying outer insulation to a bare stator bar

ABSTRACT

A method of applying an outer insulation to a bare stator bar to form an insulated stator bar. The method entails extruding at least one extruded member that will form the outer insulation. The extruded member has an opposing pair of edges that are parallel to the longitudinal length of the extruded member. In addition, the extrusion process creates an inner cavity that extends the longitudinal length of the extruded member. A bare stator bar is then inserted into the inner cavity of the outer insulation so that the outer insulation surrounds the perimeter of the bar and extends along a longitudinal length thereof. The opposing pair of edges of the extruded member are then attached together so that the perimeter of the bar is entirely enclosed by the extruded member.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of co-pending U.S. patentapplication Ser. No. 10/908,005, filed Apr. 25, 2005, which is adivision patent application of prior application Ser. No. 10/605,489,now U.S. Pat. No. 7,026,554. The contents of these prior applicationsare incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention generally relates to electric insulation materials. Moreparticularly, this invention is directed to an extruded groundwallinsulation material for a stator bar of an electric machine and aprocess for applying the insulation material to the stator bar.

FIG. 1 represents an end portion of a stator (armature) bar 10 of thetype installed in dynamoelectric machines, such as a generator used inpower generation of high-voltage alternating current. The stator bar 10can be manufactured to have essentially any length, shape and crosssection appropriate for a given generator design, voltage, and power.For most high-voltage applications, the stator bar 10 will not have asimple linear shape, but instead will have a complex shape with bendsand turns.

As shown, the stator bar 10 is composed of a number of conducting copperstrands 12 that are insulated from each other by strand insulation 13.The strands 12 are arranged to form two tiers that are separated by astrand separator 14, all of which may together be termed a “bare bar.”Surrounding the tiers is a stator bar (groundwall) insulation 15 formedby multiple wrappings of a mica paper tape 16. Typically multiple layersof tape are tightly wrapped around the conductor, usually overlapping byone-half the width of the tape, or “half-lapped.” The groundwallinsulation 15 serves to insulate the stator bar 10 from the stator inwhich it is installed.

Groundwall insulation of the type shown in FIG. 1 is widely used in thepower generation industry. The mica paper tape 16 is a prepreg composedof a mica paper typically backed by a single woven backing or a pair ofbackings. A resin composition permeates the mica paper and bonds eachbacking to the mica paper, thereby forming the prepreg tape. Examples ofthis type of groundwall insulation include commonly-assigned U.S. Pat.No. 3,563,850 to Stackhouse et al., U.S. Pat. No. 5,618,891 toMarkovitz, U.S. Pat. No. 6,043,582 to Markovitz et al., and U.S. Pat.No. 6,359,232 to Markovitz et al. After being wrapped with a sacrificialrelease film to protect the tape and prevent contamination, the statorbar 10 are placed in an autoclave for vacuum heat treatment andsubsequent curing of its tape 16. Vacuum heat treatment is carried outto remove air, moisture and any solvent or volatile compound present inthe resin binder of the tape 16 while curing under pressure serves toconsolidate the tape insulation, such that the resin binder bonds themica paper and each of its backings together to form a void-free solidinsulation. Removal of air, moisture, solvents and volatile compoundsfrom the binder is necessary to prevent formation of voids in the curedinsulation that would otherwise adversely affect the quality of theinsulation and induce premature insulation failure due to breakdownunder electrical stress. The latter characteristic of insulation istermed “voltage endurance,” and is normally due to erosion by electricaldischarge and electrochemical attack.

It can be appreciated that groundwall insulation of the type describedabove is labor intensive and incurs significant process costs.Furthermore, if not properly controlled, the taping process can lead tothe presence of voids, resulting in reduced performance reliability.Therefore, improved groundwall insulation and processes have beeninvestigated. For example, extruded groundwall insulation has beenproposed, examples of which include commonly-assigned U.S. Pat. No.5,650,031 to Bolon et al. and U.S. Pat. No. 5,710,475 to Irwin et al. Inthese approaches, the bare stator bar is passed through an extrusiondie, which deposits the groundwall insulation in-situ along the entirelength of the bar. The technical challenges associated with extrudedgroundwall insulation, including the difficulty of passing stator barswith complex shapes through a die, have been significant, such thatstator bars equipped with extruded insulation are not currently inproduction. Accordingly, there remains a demand for groundwallinsulation that overcomes the shortcomings of groundwall insulationformed of multiple wrappings of mica paper tape.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an insulated stator bar for an electricmachine, and method of applying an outer insulation to a bare stator barto form the insulated stator bar. The invention is particularly directedto groundwall insulation for stator bars used in dynamoelectric machinesthat operate at high voltages, such as a generator for power generationof alternating current delivered to a distribution network (e.g.,typically in a range of about 13.8 to 19 kV).

The outer insulation of this invention surrounds the perimeter of thestator bar and extends along a longitudinal length of the bar,preferably forming a void-free barrier surrounding the perimeter of thebar. The outer insulation comprises at least one extruded member (e.g.,a single extruded member, two individual extruded members, etc.)containing an electrical insulation material. The at least one extrudedmember comprises an opposing pair of edges parallel to the longitudinallength of the bar. The edges are joined together so that the perimeterof the bar is entirely enclosed by the at least one extruded member.

The method of the present invention generally entails the steps ofextruding the at least one extruded member that will form the outerinsulation. The at least one extruded member comprises an opposing pairof edges that are parallel to the longitudinal length of the at leastone extruded member. In addition, the extrusion process creates an innercavity that extends the longitudinal length of the at least one extrudedmember. A bare stator bar is then inserted into the inner cavity of theouter insulation so that the outer insulation surrounds the perimeter ofthe bar and extends along a longitudinal length thereof. The opposingpair of edges of the at least one extruded member are then attachedtogether so that the perimeter of the bar is entirely enclosed by the atleast one extruded member.

A significant aspect of the present invention is that extruding and thenassembling the outer insulation with a bare stator bar is simpler andless costly than prior art mica tape processes, as well as previousattempts to form an in-situ extruded insulation. Known processes such asthermoforming can be utilized to cause the insulation to conform to theouter perimeter of the bar, thereby enabling the forming of a void-freebarrier surrounding the bar.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in perspective a cross-section of a stator bar wrapped witha prepreg tape in accordance with the prior art.

FIG. 2 is a perspective end view of a two-piece extruded insulationsurrounding a stator bar in accordance with a first embodiment of thepresent invention.

FIGS. 3 and 4 are perspective views showing stator bars being insertedinto one-piece extruded insulation in accordance with second and thirdembodiments of the invention.

FIGS. 5 and 6 are partial end and plan views, respectively, of anembodiment in which mechanical supports are inserted into opposing edgesof an extruded insulation to strengthen the joint formed by and betweenthe edges in accordance with a fourth embodiment of this invention.

FIG. 7 represents a process for welding the abutting edges of anextruded insulation in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 schematically represents a cross-sectional view of an insulatedstator bar 20 for a generator of a type used in power generation ofhigh-voltage alternating current delivered to a distribution ortransmission network. The stator bar 20 includes stator bar (groundwall)insulation 24 surrounding a bare bar 22. The construction of the lattercan be the same or similar to the bare bar (12, 13 and 14) of the statorbar 10 shown in FIG. 1, though any suitable bare bar construction iswithin the scope of this invention.

The stator bar 20 of FIG. 1 differs from the stator bar 10 of FIG. 1with respect to the construction of its groundwall insulation 24. Incontrast to the insulation 15 represented in FIG. 1 as being formed bymultiple wrappings of a mica paper tape 16, the groundwall insulation 24of FIG. 2 comprises two extruded members 26 and 28. Each extruded member26 and 28 is generally C-shaped or U-shaped, which as used hereininclude the cross-sectional shapes of the extruded members 26 and 28shown in FIG. 2, namely, a base 30 and two parallel sides 32 that areeach perpendicular to the base 30. Such shapes can be readily extrudedand then subsequently assembled with the bare bar 22 to result in thestator bar 20 shown in FIG. 2.

Each extruded member 26 and 28 is represented in FIG. 2 as having amultilayer construction that includes an electrical insulation layer 34sandwiched between a pair of inner and outer layers 36 and 38,respectively. Suitable materials for the insulation layer 34 includeelectrometric and filled thermoplastic materials having sufficientlyhigh electrical resistivity. An optional but preferred property for theinsulation material is the ability to undergo thermoforming in order tomore closely conform to the bar 22. Examples of suitable filledthermoplastic materials include polysulfones, polyimides, bismaleimides,cyanate esters, polysulfides, and silicones filled with about 1 to about50 weight percent of ceramic and/or oxide particles. Suitablethicknesses for the insulation layer 34 will depend on the particularmaterial from which it is formed.

The inner and outer layers 36 and 38 may be co-extruded with theinsulation layer 34, or laminated or painted onto the insulation layer34 after extrusion. Suitable materials for the inner and outer layers 36and 38 include various conductive materials, such as those used to formconductive slot armoring and internal grading for stator bar groundwallinsulation. The presence of the conductive inner and outer layers 36 and38 is optional, but can be beneficial to allow small relative motionsbetween the conductors and the insulation layers of the bare bar 22 andto reduce any electrical stresses induced at the interface between thebar 22 and insulation 24. Alternatively or in addition, thinsemiconductive tapes may be applied inside and/or outside the extrudedinsulation layer 34. Suitable thickness for the layers 36 and 38 willdepending on the particular materials of which they are formed.

As evident from FIG. 2, each extruded member 26 and 28 has a pair ofedges 40 and 42, each opposing pair of which are shown as havecomplementary interlocking features 44 and 46. The features 44 and 46are represented as interlocking tongue and grooves 44 and 46 in FIG. 2though other configurations are possible, such as butt joints, lapjoints, etc. The interlocking features 44 and 46 serve to mechanicallylock together the opposing edges 40 and 42 of the extruded members 26and 28. The interlocking features 44 and 46 are preferably continuousalong the entire length of their respective extruded member 26 or 28,though it is foreseeable that the features 44 and 46 could bediscontinuous.

As noted above, the C-shaped cross-section of the members 26 and 28facilitate forming the members 26 and 28 by extrusion. A suitableextrusion technique would be a profile extrusion technique, by which thedifferent layers of materials are co-extruded with multiple extrusionmachines feeding into a specially-designed die head. After assembly ofthe extruded members 26 and 28 with the bar 22, members 26 and 28 formedof a thermoplastic material may undergo a thermoforming operation, inwhich both heat and pressure are applied to the extruded members 26 and28 to soften the members 26 and 28, force the members 26 and 28 toclosely conform to the exterior perimeter of the bar 22, and close gapsand voids between the extruded members 26 and 28 and the bar 22.Thermoforming can also result in a more dense material by reducing anyporosity within the extruded members 26 and 28 in the unlikely eventthat such defects are formed during the extrusion process. Suitablethermoforming techniques and parameters will depend on the particularmaterials used to form the extruded members 26 and 28, and wouldgenerally be within the skill of those familiar with thermoformingprocesses.

FIGS. 3 and 4 represent alternative embodiments of the insulation 24 ofFIG. 2, as well as techniques for their assembly with a bare bar 22. InFIG. 3, the insulation 24 is formed by a single extruded member 126having a rectangular shape that defines a central passage 128 and asingle pair of opposing edges 140 and 142 located at one of the cornersof the rectangular shape. FIG. 4 also shows the insulation 24 as formedby a single extruded member 226 having a rectangular shape that definesa central passage 228, but with a single pair of opposing edges 240 and242 located along one of the sides of the rectangular shape. In theembodiments of FIGS. 3 and 4, the edges 140/142 and 240/242 can bejoined together by welding or with interlocking features (not shown)during or following insertion of the bar 22 into their passages 128 and228.

FIGS. 5 and 6 represent features that can be incorporated into any oneof the embodiments of FIGS. 2 through 4 to mechanically secure togetherthe edges 40, 42, 140, 142, 240, and 242 of the extrusion members 26,28, 126 and 128. FIG. 5 represents a portion of the edge 40 of one ofthe extruded members 26 and 28 of FIG. 2, modified to have a pair ofslots 48 formed therein. FIG. 6 is a plan view showing the seam definedby opposing edges 40 and 42 of the extruded members 26 and 28, in whicheach edge 40 and 42 has been modified to include opposing pairs of slots48 of the type represented in FIG. 5. FIG. 6 further represents thepresence of pegs 50 received in the opposing pairs of slots 48. Aninterference fit between the pegs 50 and slots 48 creates a mechanicalinterlocking effect between the opposing edges 40 and 42 of the extrudedmembers 26 and 28. The interlocking effect can be supplemented bywelding the edges 40 and 42 together, as discussed previously.

A weld can be used with or in lieu of interlocking features placed alongthe edges 40, 42, 140, 142, 240, and 242 of the extruded members 26, 28,126, and 226. FIG. 7 represents an insulated stator bar 20 produced bythe technique of FIG. 4 undergoing a welding operation, in which a weld52 is being formed along the joint (located along one of the sides ofthe extruded member 226) using a plastic seam welding method. The weldmaterial can be a filled or unfilled resin, suitable examples of whichinclude those materials previously noted for the insulation layer 34.The welding process is schematically represented in FIG. 7 as beingcarried out with a movable weld head 54 and a fixtured stator bar 20.The weld head 54 is preferably carried on a multi-axis robotic arm (notshown) so that the weld 52 can be accurately formed along the length ofthe stator bar 20, which as depicted in FIG. 7 has a complex geometricshape.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Accordingly, the scope of the invention is to belimited only by the following claims.

1. A method of applying an outer groundwall insulation to a bare statorbar of an electric machine to form an insulated stator bar, the methodcomprising the steps of: forming a bare stator bar comprising strands ofelectrical conductors oriented parallel to each other and strandelectrical insulation surrounding each of the strands of electricalconductors; extruding the outer groundwall insulation to comprise atleast one extruded member containing an electrical insulation material,the at least one extruded member having an inner cavity extending alongitudinal length thereof and an opposing pair of edges parallel tothe longitudinal length thereof; inserting the bare stator bar into theinner cavity of the outer groundwall insulation so that the outergroundwall insulation surrounds a perimeter of the bare stator bar andextends along a longitudinal length thereof; attaching together theopposing pair of edges of the at least one extruded member so that theperimeter of the bare stator bar is entirely enclosed by the at leastone extruded member and the opposing pair of edges abut each other todefine a seam that is substantially parallel to the longitudinal lengthof the bare stator bar; and then welding the opposing pair of edgestogether with a weld material deposited at the seam by a weld head;wherein the at least one extruded member is extruded so that theopposing pair of edges comprise interlocking features that physicallysecure the edges together as a result of the attaching step, and each ofthe interlocking features is continuous along a corresponding one of theedges so as to be substantially parallel to the longitudinal length ofthe bare stator bar.
 2. The method according to claim 1, wherein theinterlocking features are formed as a projection on one of the opposingpair of edges and a recess defined in a second of the opposing pair ofedges.
 3. The method according to claim 1, wherein the opposing pair ofedges further comprise opposing slots defined in the opposing pair ofedges, the method further comprising the step of inserting a member inthe opposing slots to further mechanically secure together the opposingpair of edges.
 4. The method according to claim 1, wherein the at leastone extruded member is extruded to comprise a single extruded memberthat defines each of the opposing pair of edges, the single extrudedmember having a rectangular outer perimeter defining four corners andfour sides therebetween, the opposing pair of edges being located alongeither one of the four corners of the outer perimeter of the singleextruded member or along one of the sides of the outer perimeter of thesingle extruded member.
 5. The method according to claim 4, wherein thesingle extruded member comprises a layer of the electrical insulationmaterial and further comprises an inner layer of a conductive materialon an interior surface of the electrical insulation material and anouter layer of a conductive material on an exterior surface of theelectrical insulation material.
 6. The method according to claim 1,wherein the at least one extruded member is extruded to comprise twoextruded members, each of the two extruded members defining acorresponding one of the opposing pair of edges that are attachedtogether and defining a corresponding one of a second opposing pair ofedges that are attached together, each of the two extruded members beingC-shaped.
 7. The method according to claim 6, wherein each of the twoextruded members comprises a layer of the electrical insulation materialand further comprises an inner layer of a conductive material on aninterior surface of the electrical insulation material and an outerlayer of a conductive material on an exterior surface of the electricalinsulation material.
 8. The method according to claim 1, wherein theelectrical insulation material of the at least one extruded member ischosen from the group consisting of electrometric and filledthermoplastic materials.